This invention relates to systems for aligning the wheels of a motor vehicle, relating particularly to systems for measuring the toe and track of the vehicle wheel assemblies.
Toe and track are both related to the geometric center line of the vehicle, which is defined as the straight line extending between the midpoint of the front axle or wheel assembly and the midpoint of the rear axle or wheel assembly. Toe is the angle at which the front or steerable wheel points toward or away from the vehicle's geometric center line, i.e., it is the angle between the geometric center line and the plane of rotation of the wheel. If the front of the wheel points toward the center line, toe is positive and the condition is called "toe-in". If the front of the wheel points away from the center line toe is negative and the condition is known as "toe-out". If the wheel is parallel with the center line, toe is 0.
The thrust line is a line passing through the rear axle midway between the two wheels and perpendicular thereto. The track is true when the thrust line lies along the geometric center line of the vehicle.
Prior art alignment systems have typically been provided with two toe gauge assemblies which attach to the hubs of the wheels of a vehicle. Each such toe gauge assembly includes a housing which has mounted therein a light source and projection means for projecting a light beam onto a scale carried by the other toe gauge assembly, which is mounted on another wheel at the same end of the vehicle. In some cases each assembly also projects a second beam onto a track scale carried by another wheel on the same side of the vehicle.
In such prior systems, the heat generated from the internally mounted light source can cause distorted images and miscalibration resulting from expansion of the toe gauge assembly materials, with resultant inaccurate readings.
Also, in those systems which project both toe and track beams, it is often necessary for the user to switch the light source between the two projection directions.